1. Field of the Invention
The present invention relates to a monochrome or color image forming apparatus employing an electro photographic system, in particular to an image forming apparatus particularly using a laser diode array (herein after referred to as an LDA) as a light source.
2. Discussion of the Background Art
In the image forming apparatus employing an electro-photographic system, an optical scanning system is widely used, in which a laser beam is emitted to a rotating polygon mirror and is scanned and converges on a surface of a photoconductive member via an optical device, such as lens, etc., so that a latent image is formed.
Even though a market increasingly demands speeding up of an image forming apparatus, a number of rotations of the polygon mirror and beam pulses and a response speed of a photoconductive member almost reach a technical limit in the optical scanning system. Thus, to the speeding up, a research and development is directed to a new laser device capable of simultaneously emitting laser beams to increase a number of scanning lines as discussed in the Japanese Patent Application Laid Open Nos. 2001-264657 and 2004-276532.
The Japanese Patent Application Laid Open No. 2001-264657 discusses an array type laser head having plural light generation sections aligned to emit lights from one ends. The Japanese Patent Application Laid Open No. 2004-276532 discusses a laser head of a surface light generation type including plural light generation sections arranged in a matrix state.
However, these multi beam scanning causes a new problem of density unevenness called banding created in synchronism with a beam scan pitch. Thus, the Japanese Patent Application Laid Open No. 2004-77714 addresses reciprocity failure phenomenon mainly causing the banding by employing overlap scanning (i.e. skip scanning) as described in the U.S. Pat. No. 4,198,642.
Specifically, the Japanese Patent Application Laid Open No. 2004-77714 narrows a banding pitch caused by the reciprocity failure into a prescribed level hardly visually recognized distinctively.
Specifically, they attempted to improve image quality rather than to speed up by simultaneously writing an increased number of lines using multi beams.
To speed up, another problem need be resolved. In general, density of scanning lines ultimately formed in a sub scanning direction is determined by a feed pitch in the sub scanning direction and a number of scanning lines written per scanning. However, a feed pitch cannot be changed in a system where scanning line density in the sub scanning is fixed.
For example, when ten scanning lines are formed with density of 600 dpi (42.3 μm/line) per scanning, a feed pitch in the sub scanning direction is fixed to 0.423 mm (2.36 cycle/mm) (i.e., 0.0423 mm×10=0.423 mm (2.36 cycle/mm).
Further, density unevenness creates extraordinarily noticeable noise in this cycle. Even though overlap scanning effectively avoids the reciprocity failure, it is not able to narrow the feed pitch.
To increase a number of simultaneously written scanning lines and a depicting speed, the scanning lines need be uniform not being affected by the feed pitch rather than banding is removed from the visually noticeable range by narrowing the feed pitch.
As another serious problem that causes the banding, a scanning line bending is exemplified as discussed in the Japanese Patent Application Laid Open No. 7-209596.
Specifically, when a single beam is used, bending of a scanning line generally created due to a performance of the optical system in the sub scanning direction hardly becomes prominent. However, it easily becomes prominent while creating unevenness of a scanning line pitch when the multi beam is used and overlapped due to interference of the scanning line bending.
To resolve such a problem, the Japanese Patent Application Laid Open No. 8-227048 discloses non-integral overlap scanning, in which plural scanning lines are displaced being partially overlapping with each other.
In particular, it is described that a peak of a beam envelope is displaced in the sub scanning direction and a scanning line gap is changed while adjusting beam intensity of the overlapping scanning lines.
Further, the Japanese Patent Application Laid Open No. 6-48846 describes that for the purposes of relaxing precision needed in an optical skip scanning system and resolving a problem that abeam pitch permissible error becomes too strict, skip scanning is executed at a small spot interval as calculated by a prescribed formula.
Further, in the Japanese Patent Application Laid Open Nos. 2005-212223 and 2008-36992, it is described that when sequentially and simultaneously exposed sections are included, in which respective images are sequentially formed and overlapped with each other with a prescribed time delay, a difference of density in a neighboring pixel overlapping section is reduced.
Further, the Japanese Patent Application Laid Open Nos. 2004-109680 discusses a technology that executes exposure plural times by overlapping scanning lines plural times.
However, when multi beam scanning is executed by a highly integrated LDA having several dozens of channels as shown in the Japanese Patent Application Laid Open No. 8-227048 and it is attempted to resolve the problem of scanning line bending, increase of the number of scanning lines does not link with speeding up. Because, a number of scanning lines to be overlapped is the same as that of beams that executes simultaneous scanning as shown in FIG. 6.
Specifically, even though ten beams execute simultaneous scanning, if one raster is combined with ten scanning lines using the method of the Japanese Patent Application Laid Open No. 8-227048, substantially one scanning line is formed and cannot speed up by the multi beam scanning.
Further, a partially overlapping method as described in the Japanese Patent Application Laid Open No. 8-227048 raises a problem that scanning line bending in the sub scanning direction cannot be corrected.
Further, the Japanese Patent Application Laid Open No. 6-48846 increases scanning line density using a multi beam, but hardly contributes to speeding up.
Further, in a multi beam scanning as shown in the Japanese Patent Application Laid Open Nos. 2005-212223 and 2008-36992, neighboring pixels overlap with each other (i.e., overlapping beam spots) when simultaneous scanning (i.e., simultaneous exposure) is consecutively executed with a time delay such that a first scanning line comes again after the 20th scanning line in a twenty-beam LDA. As a result, a problem of reciprocity failure phenomenon is raised in that density changes in the overlapping section due to a different time response of the photoconductive member. Thus, such density change need be reduced.
Further, as described in the Japanese Patent Application Laid Open No. 2004-109680, a problem is raised in that increasing a number of scanning lines only increases an overlapping amount of the scanning lines, and hardly increases resolution or a depicting speed. Further, the primary objective is to maintain the banding within a space frequency and to suppress visibility thereof.